Electronic devices are greatly damaged by short circuits in the internal circuitry. Hence the casing of the electronic device provides important sealing and shielding functions to protect the internal circuit and prevent dust, moisture or external elements from directly reaching the interior. This also improves the adaptability of the device to varying operational environments, which is especially important for portable devices that are very popular these days. Waterproof and dust-proof functions have become an important design issue. In general, to provide waterproofing is more difficult since water is a fluid and can infiltrate through very small gaps and slits. Once waterproofing is achieved, the problem of fending off other external materials may also be resolved.
In the design of the casing for waterproofing, as the interface of the casing generally does not have relative movement during operation, the interface is usually coupled with washers to create forced packing to repel water. However, waterproofing for the button knob structure on the case surface is more difficult to accomplish. In order to allow the button knob to function as desired, a gap must be provided between the button cap and the case to facilitate movement of the button cap, and water tends to seep in through the gap.
To remedy this problem, numerous waterproofing designs for button knob have been proposed in the prior art. For instance, U.S. Pat. No. 5,514,843 discloses a pressure-compensated key switch that places a key cap and a switch inside a case. The key cap is extended through a key hole and exposed to receive depression. To prevent water from seeping into the case through the gap between the key hole and the key cap, an elastic membrane is deployed to cover the key hole. However, the elastic membrane directly receives compression during operation. When used for a period of time, or when hit and rubbed by external forces, the membrane is easily damaged and the waterproofing function becomes ineffective. Hence its applicability is limited. Although it can provide some waterproofing, it cannot be adapted to suit many different operation environments.